Initial support for building Yosemite guests

- In addition to growing BaseSystem and copying over Packages, we
  also copy the BaseSystem dmg and chunklist files to the root
  of this new grown BaseSystem (yes, a copy of its original at the
  root of itself).
  - My guess would be that this is so that the installer can use
    these to do recovery partition-related tasks, verification, etc.
    because without it, the installer would fail complaining that
    it could not verify the recovery partition.
  - Cribbed from the 'convert_mavericks_image' script from
    Parallels 10 - a much simpler approach than VMware Fusion's
    approach of setting up a second disk with a custom boot config.
- Fixes #17.
pkg postinstall ssh enable: new launchd overrides path on 10.10
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